The subject matter disclosed herein relates to fuel and air premixers for gas turbine combustion systems, and more particularly to a cooling system that will allow flame holding without sustaining damage to the system.
The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. One method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion, often called a Dry Low NOx (DLN) combustion system. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is significantly reduced.
There are several difficulties associated with dry low emissions combustors operating with lean premixing of fuel and air. That is, flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. Typically, there is some bulk burner tube velocity, above which a flame in the premixer will be pushed out to a primary burning zone. There is an opportunity for combustion to occur within the premixing section due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section, or auto ignition, which occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without flashback or other ignition event. The consequences of combustion in the premixing section, and the resultant burn in the nozzle, are degradation of emissions performance and/or overheating and damage to the premixing section. In other words, if a flame is held in the premixer, damage to the center body, burner tube, and/or vanes can occur in less than ten seconds, due to the extremely large thermal load.
With natural gas as the fuel, premixers with adequate flame holding margin may usually be designed with reasonably low air-side pressure drop. However, with more reactive fuels, such as synthetic gas (“syngas”), syngas with pre-combustion carbon-capture (which results in a high-hydrogen fuel), and even natural gas with elevated percentages of higher-hydrocarbons, designing for flame holding margin and target pressure drop becomes a challenge. Since the design point of state-of-the-art nozzles may reach a bulk flame temperature of 3000 degrees Fahrenheit, flashback into the nozzle could cause extensive damage to the nozzle in a very short period of time. Experimentation with high-hydrogen fuels and DLN premixers modified for these fuels exposes the difficulty of the state-of-the-art nozzles passing flame holding tests at engine-realistic conditions. A “passed” test is one in which a flame inside the premixer does not remain in the premixer, but rather is displaced downstream into the normal combustion zone.